Field of the Invention
The present invention relates to the field of compact multiband antennas for satellite aided navigation and mobile satellite communications.
Description of Related Art
Currently in the mobile satellite communication and global navigation industries there is a need for compact multiband antennas that can be easily integrated into portable devices or more generally into mobile platforms and equipment. Ideally such antennas should provide a very controlled radiation pattern, with uniform coverage of the upper hemisphere and circular polarization purity, for multipath and noise rejection. The ideal antenna should also be electromagnetically isolated from the chassis or external conductive ground structures that it is mounted on, to enable integration on multiple platforms with minimal redesign.
The fractional-turn Quadrifilar Helix Antenna (QHA) disclosed in US Patent Application Publication 2008/0174501 A1 assigned in common with the present invention, satisfies most of the above requirements. FIG. 1 shows a conventional fractional-turn QHA. Its pattern is nearly hemispherical and can be shaped to favor a particular elevation angle, if needed. Circular polarization is almost ideal over a very wide range of elevation angle. The most compact variant of the QHA has four helical elements with electrical length of about ¼ wavelength fed by a 4-port phase shifting network enforcing the proper phase rotation. A detailed description of the possible implementation of the feeding network can be found in US 2008/0174501 A1 and is omitted here.
When very compact dimensions are targeted an external matching network is necessary. The design of the matching network can be quite challenging because the strong coupling between the different arms requires that the four ports are matched simultaneously. Moreover, the design is intrinsically single band and the only way to cover multiple bands is to use as many antennas. Using multiple antennas, besides being impractical in many cases, is unacceptable in some particular applications, such as L1/L2 GPS navigation, since the difference in phase between the L1 and L2 signals needs to be accurately calibrated.